As is known, the aforementioned type of apparatuses and methods are widely used in the oil production sector.
In fact, the fluids extracted from oil wells are non-homogeneous mixtures of oil, salt water and gas, known in the sectors jargon as “multiphase fluids”.
To determine the amount of extracted oil and, in particular, the profitability of the well, it is necessary to determine with precision not only its flow rate, but also the concentration of the different phases composing the fluid.
It is known that the aforesaid parameters are determined using the so-called “multiphase” apparatuses, which include a plurality of sensors adapted to measure various properties of the fluid including, for example, the differential pressure, the density and various electrical properties.
Combining the results of the aforesaid measurements by means of correlation models and known “cross-correlation” algorithms, it is possible to calculate the concentrations and the flow rate of the different phases of the fluid.
One of the most commonly measured quantities is the dielectric permittivity, which varies as a function of the percentages of the different phases in the mixture and, therefore, is a very useful input for the determination of their respective concentrations.
As it is well known to the man skilled in the field, this measure only allows the attainment of reliable results when the oil is the continuous phase in the liquid, which normally occurs when the fraction of oil is relatively high with respect of that of water.
However, it is also known that during extraction the percentages of the different phases in the fluid, in particular the fraction of gas, are not constant but are subject to quick fluctuations.
The fraction of water also tends to gradually increase with time, according to well's exploitation.
It follows that the dielectric permittivity alone is not sufficient for a precise measurement of the concentration of the mixture moment by moment.
When the fraction of water increases, the liquid phase eventually change to a water continuous flow and the liquid becomes conductive.
The permittivity reading will in this case have to be replaced by a conductivity reading for the determination of the concentration of the different components.
In fact, the water present in the mixtures extracted from oil wells is salt water and, therefore, its electrical conductivity exceeds that of oil by several orders of magnitude.
In order to determine the precise concentration of the mixture independently of the fraction of water it contains, the known techniques make use of an electrical conductivity sensor combined with a capacitance sensor for measuring the dielectric permittivity.
In particular, one of these known techniques, disclosed in document U.S. Pat. No. 5,736,637, uses two pairs of electrodes immersed in the fluid.
The first electrode pair is in electrical contact with the fluid, so as to have a low impedance that makes it suitable to measure the conductivity of the fluid.
The second pair of electrodes is electrically insulated from the fluid and, therefore, has a high impedance and is suitable for measuring the dielectric permittivity of the fluid.
Each pair of electrodes is connected to a corresponding measurement circuit which is optimised for giving the measure of the respective electric quantity with a predetermined precision within the common range of variation of that electrical quantity.
The two pairs of electrodes are placed close to each other, so that the two electric quantities are measured in sections of the fluid very close to each other.
In order to avoid interference between the two pairs of electrodes, which would cause significant measurement errors, the circuits are activated alternatingly to each other, so that, when one circuit is active, the pair of electrodes corresponding to the other circuit is insulated.
The two measurements are executed almost simultaneously, using electronic switches which ensure high commutation frequency.
However, due to the presence of two pairs of electrodes, the aforesaid known technique poses the drawback that the sensor is particularly bulky.
The aforesaid drawback is particularly disadvantageous in the field of oil wells, where the available space is reduced and the sizes must be minimised.
The same drawback is even more evident when several electrodes are used in combination, in order to being able to use cross-correlation algorithms to determine the speed of the fluid.
The presence of two pairs of electrodes implies the further drawback of doubling the number of connections between the electrodes and their respective measurement circuits, causing the device to be more complicated and the risk of failures to increase.
In some subsea installations where the requirements for redundancy is high sometimes three or even four sensors are used, which further worsen the above mentioned drawbacks.
In addition, although the two pairs of electrodes are close to each other, the respective measurements are executed in two different zones of the pipe which, due to the erratic composition of the mixture, generally contain quantities of fluid having different composition.
Consequently the correlation of the two measurements, necessary to establish the global electrical properties of the fluid from moment to moment, introduces errors that limit the precision of the measurement.
In the attempt to overcome the aforesaid drawbacks, another known device makes use of three electrodes rather than four, two of which are transmitter electrodes while the third acts as a common receiver electrode and is kept virtually earthed.
One of the two transmitters is in electrical contact with the fluid while the second is insulated with respect to it.
This way, the first electrode has a lower output impedance with respect to the second electrode, so that the two electrodes coupled with the receiver can measure the electrical conductivity and the dielectric permittivity of the fluid, respectively.
The two transmitters are fed with different signals, for example in quadrature phase or with different frequencies, so that it is possible to separate the signals coming from each transmitter to allow determination of permittivity and conductivity values.
This device has a lower number of electrodes than the preceding device but, nonetheless, it requires the use of a specific transmitter electrode for each electric quantity, thus being partially subject to the aforementioned drawbacks.
In addition, both the aforesaid known devices pose the drawback of not allowing a precise measurement of the electrical conductivity of the fluid.
In fact, the measured resistance between the electrodes also includes the impedance at the interface between the electrodes and the fluid, called “transfer impedance” in the technical jargon, which is not known beforehand and, therefore, causes measurement errors.
Another disadvantage of both known devices described above lies in that they are subject to an error in the permittivity measurement.
In fact, the electrodes used to measure the conductivity introduce a stray capacitance that could, in some cases, affect the permittivity measurement.
Document U.S. Pat. No. 5,216,409 further discloses a device for detecting contaminants in an alcohol-gasoline mixture for internal combustion engines. This device uses a single sensor, which is switched between different circuitry for measuring both permittivity and conductivity.
Document CH 326 215 discloses an apparatus for displaying the properties of softening solutions of water treatment plants by measuring the conductivity of the solution through an electrode pair.